Plasma processes employed in semiconductor fabrication are constantly being improved in order to make smaller device feature sizes in thin film structures on semiconductor wafers. Currently, feature sizes are in the range of tens of nanometers. The ever decreasing feature sizes are difficult to realize without accurate control of delivered RF power. The amount of RF power delivered to the plasma is affected by fluctuations in plasma impedance. Such fluctuations are typically compensated by a conventional impedance match element or circuit. One problem is that impedance match elements or circuits have a significant delay in responding to plasma impedance changes. For example, a variable reactance impedance match circuit has a response delay on the order of a second, typically. A tuned frequency impedance match system has a response delay on the order of 100 msec. Random or sporadic fluctuations in plasma impedance occurring at rates faster than the impedance match response delay may cause the impedance match to fail, destroying control over delivered RF power to the plasma. Moreover, an impedance match circuit has a limited match space or range of plasma impedances over which the match is able to maintain the load impedance presented to the RF generator sufficiently close to 50Ω to maintain the voltage standing wave ratio (VSWR) at the RF generator output below a threshold above which the generator does not function.
In the presence of random fluctuations in plasma impedance with a rise time corresponding to 100 kHz, the RF impedance match circuit has difficulty following the rapid plasma impedance change, and may cease to function properly, so that it creates an impedance mis-match. Upon this occurrence, the power reflected back to the RF generator exceeds an acceptable level, and the reactor is shut down.
The inability of the impedance match circuit to follow the higher frequency transients may be attributable to its design. For impedance match circuits employing variable reactance elements, the variable reactance elements may have mechanical limitations that slow their response, and typically have response times on the order of one second. For impedance match circuits employing tuned frequency generators, the frequency tuning element of such a device may have mechanical limitations that slow their response, and typically have response times on the order of 100 milliseconds.
The action of the RF impedance match circuit in maintaining a constant impedance match for the RF generator is necessary for two reasons. First, the measurement and control of RF power delivered to the plasma must be sufficiently accurate to carry out requirements of the process recipe. Secondly, the RF generator must be protected from damage by reflected RF power (which is caused by an impedance mis-match between the RF generator output and the plasma).